1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting element with improved luminous efficiency of an active layer, and a method for manufacturing the same. The present invention further relates to a self-pulsating nitride semiconductor light emitting element and a method for manufacturing the same.
2. Description of the Related Art
A nitride semiconductor light emitting element has an oscillation wavelength of approximately 400 nm, and it has been developed for an optical disk system. Further, since it can withstand up to high output, its application is widely studied in the fields such as a high-power light source, a pulse oscillator and the like. To be used in various applications, the important issue is to lower the oscillation threshold value. Thus, various improvements, such as improvement of the substrate, the epitaxial growth technique, the multiple quantum well active layer and the like have been introduced. For example, Japanese Patent Laying-Open No. 8-228025 discloses a semiconductor light emitting element having a structure shown in FIG. 14 as a conventional improved nitride semiconductor light emitting element.
As shown in FIG. 14, the light emitting element has a structure in which a buffer layer 452, an n-type contact layer 453, a second n-type clad layer 454, a first n-type clad layer 455, an active layer 456, a first p-type clad layer 457, a second p-type clad layer 458, and a p-type contact layer 459 are successively stacked on a sapphire substrate 451. By providing first n-type clad layer 455 or first p-type clad layer 457, the crystallinity of active layer 456 is improved to attain higher luminous efficiency.
However, some characteristics can be found when the nitride semiconductor light emitting element according to the conventional technique is operated at most at the oscillation threshold value to observe the emission spectrum from the backside of the wafer with a spot size of several μm. When measurement is made with different spot positions in a laser resonator, the wavelength that attains the maximum intensity varies and often the full width at half maximum of the emission spectrum is found to be wide or a sub peak is observed in the long-wavelength region of at least 440 nm. Such an nitride semiconductor light emitting element is higher in oscillation threshold value due to lower stimulated emission probability, as compared to a nitride semiconductor light emitting element having a unimodal emission spectrum with narrow full width at half maximum.
Further, Japanese Patent Laying-Open Nos. 8-228025, 9-266327 and 11-330614 already disclose light emitting elements in which a conventional n-guide layer formed of a nitride semiconductor containing In and Ga is provided to suppress degradation of crystallinity, which may be observed when an active layer formed of InGaN is provided on a layer formed of GaN or AlGaN. However, the inventors of the present invention have thoroughly studied these structures and found that they do not provide satisfactory effect.
When a nitride semiconductor laser is used as a light source for an optical disk system or the like, a problem associated therewith is an optical feedback noise resulting from a light being reflected at the disk surface and coupled again with the semiconductor laser. Generally, it is known to lower the coherence by bringing the carrier density of the semiconductor laser into a transient state to alleviate the gain concentration of the oscillation spectrum. To this end, the high-frequency superposition for modulating the injection current, or the self-pulsation using the interaction of carriers and photons in the semiconductor laser may be carried out. Specifically, the self-pulsation is more advantageous in the viewpoint of cost and ease of use, since it does not require to use high-frequency circuitry.
Such a self-pulsating nitride semiconductor laser is disclosed, for example, in Japanese Patent Laying-Open No. 9-191160. This laser is a low-noise semiconductor laser for an optical disk shown in FIG. 21, which serves as a stable low-noise semiconductor laser by including a saturable absorption layer having InGaN as a constituent element. The structure of the nitride semiconductor laser is as follows. Referring to FIG. 21, an n-type AlN layer 701, an n-type AlGaN clad layer 702, an n-type GaN light guide layer 703, an InGaN quantum well active layer 704, a p-type GaN light guide layer 705, a p-type AlGaN clad layer 706, a p-type GaN contact layer 707 are successively stacked on an n-type SiC substrate 700. Additionally, p-type GaN light guide layer 705 is provided with an InGaN saturable absorption layer 708.
Further, Japanese Patent Laying-Open No. 9-191160 discloses that an InGaN saturable absorption layer may be provided to n-type GaN guide layer 703. Generally in a nitride semiconductor laser, a layer between a substrate and an active layer (hereinafter referred to as “an active layer lower layer”) is n-type while an active layer upper layer is p-type. A saturable absorption layer includes an n-type saturable absorption layer provided in an n-type layer and p-type saturable absorption layer provided in a p-type layer.
The nitride semiconductor laser is formed of a clad layer mainly containing Al and Ga and an active layer mainly containing In and Ga, while the saturable absorption layer that absorbs laser light is also formed of a layer mainly containing In and Ga. The layer containing Al and Ga, or the layer mainly containing Ga is higher in the growth temperature than the layer mainly containing In and Ga by at least 100° C. normally, and in some cases by nearly 300° C. Therefore, it has characteristics that the temperature during a sequence of epitaxial growth varies greatly. Since such temperature varie may degrade the state of the active layer and promote segregation of In, the growth procedure must be carried out carefully when providing the saturable absorption layer.
When In segregation of the active layer occurs, self-pulsation is hindered since gain is aggravated. Further, with the conventional structure, self-pulsation may partially be hindered, or the light output in which self-pulsation is observed may not be stable. Still further, with the structure of Japanese Patent Laying-Open No. 9-191160 also, an adverse effect due to thermal hysteresis tends to occur since the GaN layer or the AlGaN layer is provided between the InGaN active layer and the InGaN saturable absorption layer.